1. Field of the Invention
The present invention relates to a transparent coating having a medium refractivity, consisting substantially of at least one metal oxide mixture. It relates further to a method of producing a transparent coating, with an optical substrate having at least one reflection reducing transparent coating, and to a reflection reducing multi-layer coating for the reduction of the reflection at an optical substrate, including at least one transparent coating or layer, of a medium refractivity, consisting of at least one metal oxide mixture.
2. Description of the Prior Art
Thin oxide layers are widespread in technical engineering matters as protective layers and as layers for optical purposes. In the case of protective layers, they serve to protect sensitive surfaces of bodies, e.g. precision parts of precision engineering products, of lenses, surface mirrors or similar structures against corrosion and mechanical damage. In the optical industry oxide layers are additionally used as high and low refractivity layers for reflection reducing coatings, further for interference filters, ray dividers, heat filters, cold light mirrors, coatings for eyeglasses and similar items. The mechanical and optical properties of such oxide layers do not only depend on the kind of the oxide which has been deposited but rather exeedingly on the kind of the depositing method. Accordingly, e.g. optical glasses having a refractivity generally in the range between 1.4 and 1.8 are given reflection reducing coatings for a reduction of the reflection losses.
For the treatment of optical substrates such as lenses, prisms, filters, etc. the earlier predominantly deposited single and double layers are superseded due to the introduction of process controlled plants also in the mass production by the qualitatively better multi-layers. The substrates of the optical field which are to be treated are generally anorganic and organic glasses which encompass a large range of refractivity in the magnitude of n=1.4 to 1.8. In contrast thereto, multi-layer systems for the reduction of losses of the refractivity display their optimal effect merely in a relatively narrow refractivity range. For this reason the above mentioned total range has been divided into two portions, a so called low refraction range below n=1.6 and a higher refraction range with n=1.6. For both range portions different layer systems were used accordingly for the reduction of the reflection.
The division of the refraction range into two range portions is, however, quite coarse and because the reflection reducing effect still occured optimally in both range portions only in narrow portion parts, it was necessary to come to compromises for substrates with refractivities at which no optimal reduction of the reflection was possible.
It also has been found that when using two layer systems the vapor depositing apparatuses had to be converted and differing method steps followed which obviously is undesireable regarding production techniques. Quite often such lead to errors and a reduction of the quality.
Following these shortcomings there is thus the desire to equip optical substrates with reflection reducing multi-layers whereby in order to optimize the reflection reducing effect this multi-layer is to be adjusted by simple procedures to the refractivity of the optical body to be coated such that the above mentioned technically unsufficient solution must no longer be used.
Refection reducing multi-layer coatings are known since a long time. The U.S. Pat. No. 3,185,020 discloses a triple-layer coating of the type .lambda./4-.lambda./2-.lambda./4 whereby a .lambda./4 layer is to be understood as being the product of thickness of layer x refractivity, where the optimal thickness of the layer amounts to one quarter of the reference wave length.
The known multi-layer coatings encorporate the problem that dependent from the refractivity of the substrate the layer closest to the substrate must have a refractivity in the range of 1.65 to 1.8. Substances having good mechanical properties and incorporating this refractivity are, however, not known.
In order to solve this problem differently use was for instance made of the known fact that it is possible to substitute a symmetric triple-layer system for a .lambda./4 layer of arbitrary refractivity. Accordingly, multi-layer systems are arrived at whereby the refractivity of the composite .lambda./4 layer is adjustable by the thickness of the layer of the individual layers. The drawback of this possible solution is that when producing a plurality of layers an increased number of error sources is present at any rate and that problems of condensation can arise at the partly very thin part layers, and that due to the larger number of layers the duration of the charge is increased.
In accordance with the proposition to deposit mixtures of high and low refractory substances the German published patent application DE-AS 21 54 030 disclosed for instance a multi-layer coating in which mixtures of cerfluoride and ceroxide or mixtures of cerfluoride and zinc sulfide are used. The literature discloses further mixtures of ceroxide and silicon oxide, zinc sulfide and magnesium fluoride, zinc sulfide and natrium aluminum hexafluoride and ceroxide and magnesium fluoride. Because all of these known systems include, however, at least one component which is known not to be satisfactory regarding mechanical properties and stability, the described systems are not suitable for the production of reflection reducing multi-layer coatings having the required qualities, specifically regarding mechanical properties and stability.
General information regarding the use of layers of oxide mixtures are, furthermore, to be found in the DE-OS 20 50 556, DE-OS 24 57 474, DE-OS 28 27 856, U.S. Pat. No. 3,604,784 and GB-PS 13 80 793. All these disclosures describe, however, high refractive .lambda./2 center layers of a homogeneous structure and accordingly non-changing refractivity. Propositions for an optimizing of the reduction of the reflection by at least one layer of medium refractivity located closest to the substrate are not contained in any of these disclosures.
The DE-OS 27 20 742 discloses also a multi-layer coating consisting of a combination of Al.sub.2 O.sub.3 -layers with NdF.sub.3 and MgF.sub.2 -layers. Oxide-mixtures for these layers are, however, not described.
The DE-PS 30 09 533 discloses a coating which consists of a mixture of tantalum oxide and aluminum oxide and includes the requisite refractivity. This mixture is conditionally suitable indeed, has however in spite thereof some deficiencies. Because tantalum has a relatively low affinity to oxygen it is quite difficult to completely oxidize same during the vapor deposition. Conclusively, a relatively high partial pressure of O.sub.2 must be selected. It has additionally been found that the refractivity changes during the vapor deposition and this is undesirable.
The DE-PS 1 228 489 discloses also a oxide mixture of titanium oxide and oxides of the group rare earths which, however, leads apart from an excellent oxidizability specifically to a high refractivity n&gt;2.